I searched for todo management for years and I tested tons of apps. But as a developer this is the solution I use that works well for me:

Use YAML as a todo list, you can organise your things like:

Perso:

  - Take an appointment at ...

  - ...

Client1:

  - Do something

  - ...

Then put it in todo.yml in your Google Drive (for backup):

/Users/mishaker/Google Drive/My Drive/todo.yml

Then add an alias on your cli:

alias todo="subl /Volumes/GoogleDrive/My\ Drive/todo.yml"

Voilà, that's a simple solution which works for me.